1. Field of the Invention
The invention relates to the field of active power filters and in particular to a control method based on one-cycle control.
2. Description of the Prior Art
In recent years, the usage of modem electronics equipment has been widely proliferating. This electronics equipment imposes nonlinear loads on the ac main or source that draw reactive and harmonic current in addition to active current. The reactive and harmonic current results in a low power factor, low efficiency, harmful electromagnetic interference to neighborhood appliances, as well as overheating of transformers. In order to solve these problems, many international agencies have proposed firm harmonic restrictions to electronic equipment. As a result, a vast number of power factor correction (PFC) techniques have been proposed to comply with these regulations.
Most techniques use a current shaper, whether in a two-stage multiple-switch configuration or a two-stage one-switch configuration, to shape the input current to a sinusoidal waveform. Since the current shaper is in the series path of the power, it requires high current and high voltage semiconductor devices and entails significant power losses. Therefore, PFC techniques are generally suitable for low to medium power applications. Furthermore, it is not convenient to insert a current shaper in existing electronic equipment, since significant redesign would be required. In high power applications, a parallel harmonic correction technique using an active power filter (APF) has been proposed and explored by many researchers. See, Fabiana Pottker and lov Barbi, "Power Factor Correction of Nonlinear Load Employing a Single Phase Active Power Filter: Control Strategy, Design Methodology and Experimentation" PESC 1997 Record 28.sup.th annual IEEE Power Electronics Specialists conference; D. A. Torrey, A Al-Zamel, "Single-phase active power filter for multiple nonlinear loads" IEEE Transactions on Power Electronics, Vol. 10, pp.263-271, May 1995; Simone Buso, Luigi Malesani, "Comparison of Current Control Techniques for Active Filter Applications" IEEE Trans on Industrial Electronics. Vol. 45. No. 5 October 1998; J.-C. Wu and H.-L. Jou "Simplified control method for the single-phase active power filter" IEE. Proc. Electr. power., Vol. 143, No. 3, May 1996; Hirofumi Akagi, "New trends in active filter for improving power quality" Proceeding of the 1996 International Conference on Power Electronics, Drives and Energy System for Industrial Growth; and J. Sebastian Tepper, Juan W. Dixon "A simple-frequency-independent method for calculating the reactive and harmonic current in a nonlinear load" IEEE Transaction on Industrial Electronics, Vol. 43, No. 6, December 1996.
An APF is a device that is connected in parallel to and cancels the reactive and harmonic currents from a group of nonlinear loads so that the resulting total current drawn from the ac source is sinusoidal. Ideally, the APF needs to generate just enough reactive and harmonic current to compensate the nonlinear loads in the line, thus it handles only a fraction of the total power to the load. Most APF control methods previously proposed need to sense the line voltage and the nonlinear load current, and then manipulate the information from these sensors to generate a current reference for the APF. Since the reference current has to reflect the load power of the nonlinear load, a multiplier is needed to scale the magnitude of the current reference. A control loop is necessary to control the converter to generate the reactive and harmonic current required by the nonlinear load. These functions are generally realized by a digital signal processing (DSP) chip with fast analog-to-digital (A/D) converters and high-speed calculations. The performance of these active power filters is based on three basic design criteria: the converter topologies, the control method used, and the method used to obtain the current reference. The complex circuitry results in high cost and unreliable systems, preventing this technique from practical applications.
What is needed is an API not subject to the inherent disadvantages of prior designs.